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Hajimohammadi H, Zabbah S, Hatami J, Ebrahimpour R. Perceptual decision-making in the presence of natural stimulus: A behavioral study. Advances in Cognitive Sciences 2021; 23 (3) :119-134
URL: http://icssjournal.ir/article-1-1264-en.html

Introduction
Perceptual decision-making studies have widely focused on simple stimuli with uniform distribution of spatial information, say Random Dot Motion to address the accumulation of sensory evidence resulting in a binary decision. However, the real-world stimuli are mostly complex containing a non-uniform distribution of spatial information. Hence perceptual decision-making process in the presence of natural stimuli has remained unknown, likely because of the complexity in controlling the multiple pieces of information over time to study their effect on decisions. Recently, a novel framework has been introduced to broaden perceptual decision-making studies into multi-spatial stimuli by a face discrimination task with controlled fluctuating local features (eyes, nose, and mouth) as multiple spatial information. Another importance of this task is to bridge the perceptual decision studies and object recognition, which will be expected to improve the understanding in either area. Using the main essence of this framework, we design our own experiment by applying more control on the spatial information resulting in a decision. In fact, this experiment design consists of both the single spatial information and multiple ones. This study aims to investigate the effect of each piece of spatial information on the decision individually, as well as to study the effect of combining this information on the decision. Consequently, this experiment design enables us to quantitatively predict the behavior of multiple informative trials from single ones. In particular, this research tends to study the combination and interaction of single information and their weights to address behavior in the presence of multiple spatial information in making a binary decision. This research hypothesizes that the perfect accumulator assumption might explain the integration of spatial information.
Methods
Having developed a customized algorithm, neutral human faces have been geometrically morphed such that -100% and +100% represent the first prototype, the second one, respectively. 0% morph depicts the middle face, which is half of the morph stream between the prototypes. The trials were designed to stimuli either have three informative features (eyes, mouth, nose simultaneously) or only one informative feature (eyes, mouth, and nose individually) fluctuating in time. These stimuli enabled us to control the featural information independently to study the impact of multiple spatial and temporal information in making the decision. The 3-informative and 1-informative trials were randomly interleaved in a block. Seven human subjects (24-39 years old, four women) participated in this study with normal or corrected-to-normal vision. All procedures were approved by the Ethics Committee of the Iran University of Medical Sciences. Stimuli were created and controlled in MATLAB and presented using routines from the psychophysics Toolbox extension. The subjects got familiar with the prototypes before taking part in the main experiment and also the structure of the task and its environment. After passing the training phases by reaching the required accuracy and speed, the participants were invited to the main phase of the experiment. They were instructed to maintain their gaze on the red fixation point in the center of the screen and report if the stimuli displayed on the side of the fixation point were more similar to the first prototype or the second one. Each participant completed a total of 1568 trials on average in eight blocks of 196 trials.
Results
Logistic regression analysis revealed that the subjects were not biased toward any of the choices. The reaction time decreased, and the probability of correct answers increased with the stimulus strength (morph %), showing that the subject confirmed their decision as expected based on the strength of perceived evidence. Another measure of the accumulation of evidence that this design of the experiment allowed us to compute was the psychophysical kernels for 0% morph trials. The positive amplitudes of the psychophysical kernels indicated that the decisions are made based on the integration of spatial information yet with unequal weight. Eyes weighed more for both 3-featural, which is inconsistent with the previous findings for neutral faces and 1-featural trials. The averages of psychophysical kernels amplitudes for only-nose and only-mouth trials were positive but not statistically significant, while these averages in 3-featural trials were calculated statistically significant. This finding showed that the presence of eye information in 3-featural trials intensified the extraction of information from nose and mouth in consistency with the reliability of the information in multiple sources of information studies. To quantify the contribution of each feature in the decision, we used logistic regression. The results showed that only linear coefficients relating to eye, nose, and mouth information have a statistically significant effect on the decision rather than multiplicative terms representing the interaction of features. The psychometric function and psychophysical kernels led us to use the integration model to predict the probability of corrects achieved in 3-featural trials from the evidence calculated from the 1-featural trials. To this end, the evidence contained in each one-informative feature tria was calculated and linearly summed up with each other based on the regression model finding. Therefore, using the perfect linear accumulation assumption, we computed the expected accuracy from the result of 1-informative feature trials to predict the accuracy in three-informative feature trials. The present study showed that only considering the effective weight for each feature could extend the perfect accumulator assumption to explain the experimental accuracy for three-informative feature trials.
Conclusion
The present study's findings suggest that the presence of multiple featural information simultaneously improved the extracting of information from less reliable features. This was shown by statistically significant weights of the nose and mouth in psychophysical kernels of 3-informative feature trials compared to 1-featural trials. Furthermore, the mechanism of perceptual decision-making in the presence of natural stimuli can be quantitatively explained by an extension of the perfect accumulator assumption by considering the informative featural weights. Considering the effective weight of each feature, the perfect accumulator assumption explains such a decision-making process in the presence of complex natural stimuli. This also might address the holistic effect quantitatively. Finally, taking advantage of perceptual decision-making findings can enhance our understanding of natural stimuli processing by paving the way for using neural imaging for further investigation of brain activities during such complex processes.
Ethical Considerations
Compliance with ethical guidelines
The present study consisted of experimental protocols approved by the Ethics Committee of the Iran University of Medical Sciences (IR.IUMS.REC.1399.1281). Written informed consent was obtained from all participants. The collected data would remain confidential and would be used only for scientific purposes. The participants were allowed to leave the experiment at any phase.
Authors’ contributions
Hadiseh Hajimohammadi: conceptualization, data collection, analysis, visualization, writing original draft- review, and editing. Sajjad Zabbah: conceptualization, supervision, writing review, and editing. Javad Hatami: Supervision, writing-review and editing. Reza Ebrahimpour: conceptualization, supervision, writing review, and editing.
Funding
This study was funded by Iran Cognitive Sciences & Technologies Council (9990). Data were recorded in the Cognitive Science Laboratory of Shahid Rajaee University.
Acknowledgment
We would like to thank all the participants in this study. The special thanks go to knowledgeable colleagues and open science for providing us helpful comments and technical supports where it was needed.
Conflict of interest
The authors declare no conflicts of interest.